The present invention relates to a coil system for a magnetic resonance imaging (MRI) system. An example of such an MRI system is the Toshiba MRT-50A NMR scanner. In such device, a patient is placed upon a table which includes a plurality of linearly arranged surface coils. A transmit coil induces a high power R.F. magnetic field in the patient during a "transmit" mode, which causes precessing magnetization in the patient. The surface coils detect this magnetization during a "receive" mode and output electric signals which may be processed by a computer associated with the MRI system.
Surface coil arrangements are used in MRI systems because they are advantageous in imaging anatomical regions close to the body's surface. For example, a very small coil near an eye can give a much better signal/noise ratio output signal than a larger coil which surrounds the entire head. In general, small coils are sensitive to signals from shallow depths in the patient over an area limited by the area of the coil. Thus, if an extended region or a deep region of the patient is to be imaged, a larger coil is necessary, which results in increased background noise levels.
A compromise between the volume of sensitivity of a coil and the resultant noise level in the output signal is ordinarily selected by an operator who selects appropriate coils from an arsenal of different coils, according to the region to be imaged. The coils are then installed in the MRI system before the patient is scanned. For example, a specific region of interest for surface coil imaging is the human spine. An operator may wish to image a long sagittal plane through the center of the spinal column to locate a suspected problem. Once the problem has been located, the operator may wish to image a transaxial slice through that location to analyze the problem. The long sagittal scan requires a long rectangular coil, but the small transaxial scan requires a much smaller square or circular coil. This is particularly disadvantageous in that the patient, who is lying supine upon the coils, must be moved while the coils are changed, which is time consuming (expensive), uncomfortable for the patient (who probably has back problems), and may result in an inaccurate relocation of the scan region.
One solution is to provide a ladder-shaped coil array in which an operator switches "rungs" of the ladder-shaped array to determine the overall rectangular size of the coil as necessary. Such an arrangement has limitations in that the effective position of the coil is difficult to change because the portion of the ladder which includes the connection terminals must be part of the selected rectangle. Alternatively, two different coils may be disposed in the same location. This technique is an unacceptable compromise because it results in parasitic coupling between the coils, which degrades the electrical performance of the coils.
Some devices which include a plurality of coils have reduced coupling therebetween by partially overlapping the coils to make the mutual inductance zero. Such overlapping coils are difficult to fabricate, however, and may give an inappropriate total response pattern. Further, this arrangement adequately reduces the coupling only between adjacent coils while mutual inductance may still exist between alternate coils in the array. As an alternative approach, the effect of coupling may be reduced using feedback techniques to "damp" the effective Q of tuned circuits used in the coil configuration. This, of course, complicates the tuning of the coil and any related amplifier designs.